1. Field of the Invention
The present invention relates to a control system for an automatic transmission.
2. Description of the Related Art
Conventional automatic transmissions include a gear train, with rotation input from the engine to one gear element of the gear train and output from another gear element, and a plurality of frictional engagement elements, such as clutches and brakes, which are applied or released to transmit the rotation selectively between the individual gear elements, thereby establishing various gear stages by operatively combining different gear elements.
The frictional engagement elements are individually equipped with corresponding hydraulic servos. A shift to a predetermined gear stage can be effected, for example, by feeding oil to a hydraulic servo for a frictional engagement element which is not engaged and by draining oil from a hydraulic servo engaging its corresponding frictional engagement element.
In case, for example, the vehicle is to be run at a 2nd speed in D-range, the brake for the D-range is applied. However, if the vehicle is to be run at 2nd speed while effecting engine braking, the engine braking brake is applied in addition to the D-range brake.
Specifically, for running the vehicle at 2nd speed in the D-range, a sun gear to be rotated backwards is locked, but the torque necessary for this locking is low and thus it is sufficient to apply the D-range brake only. However, if the vehicle is run at the 2nd speed while effecting engine braking, not only the torque for locking the sun gear but also a reverse torque for effecting the engine brake is required so that the total torque necessary becomes extremely high. As a result, not only the brake for the D-range but also the brake for engine braking must be applied.
There has been proposed (in Japanese Patent Publication No. 40576/1992) a control system for an automatic transmission, in which brake capacity is changed according to the engine load by applying not only the brake for the D-range, but also the brake for engine braking, when the engine load is high, and by applying the brake for the D-range only when the engine load is low. Thus, the number of friction discs of the D-range brake is reduced to make it possible to reduce the size, to suppress the shift shock and to improve the durability of the friction discs.
In the control system of the prior art, however, the shifting time is prolonged because the two brakes are to be simultaneously applied. Specifically, when a clutch or brake is to be applied, the corresponding hydraulic servo is fed with oil through an orifice, and the oil pressure (hereinafter, the "apply pressure") in the hydraulic servo is raised under control of an accumulator. However, when two brakes are to be simultaneously applied, as described above, the two hydraulic servos must both be fed with the oil. This prolongs the time required for the individual hydraulic servos to be filled with oil and, accordingly, prolongs not only the period of time from the shift decision to the start of brake application, but also the period of time from the start of brake application to the end of application. As a result of the prolonging of the shift time period, the shift feel is poor.